Lunfardo was the 'Argot of the thieves' in early 1900's at the River Plate zone (Buenos Aires and Montevideo), and today is a non-structural deviation from the Argentine Spanish Dialect.

Origin:

Argentina is a country that promoted immigration. This immigration came with the wars in Europe (WWI and WWII), forming different colonies of immigrants that grouped themselves into separate settlements. Immigrants from Italy, Spain, France, Germany, Wales, Scotland, Ireland, England, The Netherlands, Hungary and Poland, helped to form the vast dictionary of Lunfardo's words.
These immigrants needed to adapt themselves to the local language, which they slowly did, but added their own words to the 'daily dictionary'. Soon, the rest of the population started to use these words, and created an argot that was used in the 'Arrabales' (lower class neighborhoods) by the thieves or 'Malandras' of that area.

Expansion and Evolution:

Lunfardo has become a 'daily-growing-lingo', and nowadays transformed the way of speaking of the Argentines. It is not only available in Buenos Aires, it also might be found in Montevideo, Rosario, Santa Fe, Cordoba and Entre Rios in a lower degree. There are two types of Lunfardo: Lunfardo Antiguo (Old Lunfardo, Tango) and Lunfardo Moderno (Modern Lunfardo).

Structure:
Lunfardo has exactly the same structure of Spanish, but with a good percentage of words that do not belong to any language, but have etymologies in different languages. (See Example 1)

Note the words Andamio (Andiamo, from Italian 'to go'), Laburar (Lavorare, from Italian 'to work') and Rope (Perro, from Spanish 'dog'), come from different languages and/or are written in Vesre (Reves, reversing the syllabes), which causes unnoticed people not to understand what the speaker is about to.
However, there are words that carry no etymology, and cannot be explained without relating it directly to the subject that that word is referring to.
For Example: Mina. Mina has no etymology, and cannot be related to any other word in any other language with a grade of sense. However, in Lunfardo, Mina means 'young-woman'.